Ethernet is a local area network (LAN) technology that allows transmission of information between computers at speeds of 10 million bits per second (Mbps), 100 Mbps (Fast Ethernet) and, more recently, one gigabit (×109) per second. Ethernet assumes that an unlimited number of devices can be connected on a shared transmission medium such as a thick coaxial in the original 10 Mbps Ethernet, as well as thin coaxial, twisted-pair, and fiber optic media in the newer versions. To control the exchange of data between the stations connected on the shared medium, Ethernet uses a protocol called CSMA/CD. This stands for ‘Carrier Sense, Multiple Access, Collision Detect’. The ‘Multiple Access’ part means that every station is indeed connected to the shared transmission forming a single data path. The ‘Carrier Sense’ part says that before transmitting data, a station checks to see if any other station is already sending something. If the transmission medium appears to be idle then, the station can actually begin to send data. However, two stations can start transmitting at the same time, causing a collision. When this occurs, each interfering station is made able to detect it. Hence, all stations attempting to transmit back off, and try a retransmission at randomly selected later times, thus minimizing the chance of another collision.
This simple mechanism to implement LANs has had great success and has been universally adopted. Indeed, a majority of installed LANs throughout the world are actually Ethernet LANs follow the corresponding IEEE (Institute of Electrical and Electronics Engineers) standard i.e., IEEE 802.3.
Although Ethernet does not set an upper limit to the number of stations that can be connected on a same transmission medium there are, in practice, drastic limitations. Generally speaking, as more users are added to a shared network or, as applications requiring more data are added, performance inevitably tably deteriorates. This is because all users become competitors in trying to use a common resource: the shared transmission medium. It is generally agreed that, on a moderately loaded 10 Mbps Ethernet network being shared by 30–50 users, the network can only sustain throughput in the neighborhood of 2.5 Mbps after accounting for packet overhead, inter packet gaps and collisions resulting from the use of the here above CSMA/CD protocol. Thus, although simple, the CSMA/CD protocol has limitations in its ability to take advantage of the intrinsic performance of the shared transmission medium i.e., 10 Mbps in this example. Further increasing the number of users (and therefore packet transmissions) creates an even higher collision potential. Since collisions occur when two or more stations attempt to send information at the same time, when stations realize that a collision has occurred, they must, to obey standard, all shut off for a random time before attempting another transmission. This tends to add a considerable overhead, severely impacting performance, until the mechanism just collapses when the shared transmission medium is attempted to be too much utilized.
One well-known solution to alleviate this problem is to segment traffic over independent, disjoint, smaller collision domains, however at the expense of having to put in place extra devices to allow communication between the independent pieces thus created of a LAN. This may be a bridge, a hub or a switch. For example, an eight-port high-speed switch can support eight Ethernets, each running at a full 10 Mbps so as to be able to interconnect more users on what appear however to them as a single LAN. Thus, at the expense of creating a more expensive and complicated network which goes against the original objective of the Ethernet LAN to be a very inexpensive solution, simple to deploy and to administrate for local communications typically over a campus or between the employees of a company dispersed over a group of buildings.
Another LAN technology is Token Ring. It is described in IEEE standard 802.5 and based on the circulation of a token between stations. Each station must wait for the token in order to be authorized to transmit on the shared medium here referred to as a ring, thus completely solving the collision problem here above discussed. Indeed, this has proved to allow a better utilization of the shared medium. On the other hand, token ring technology is more expensive, more complex, and therefore less widespread in today's LAN networks.
Thus, it is a broad object of the invention to provide an improved, collision-free, Ethernet technology.
It is another object of the invention to allow this improved Ethernet to be useable over the current inexpensive, highly utilized, and well-known CSMA/CD Ethernet network infrastructure.
It is a further object of the invention to permit the passing-token mechanism of the Token Ring LANs to be efficiently carried out over existing Ethernet networks so that a physical Ethernet network becomes collision free, and therefore can be utilized at higher rates.
It is yet another object of the invention to permit collision and collision-free mechanisms to coexist on a single Ethernet network.
Further objects, features and advantages of the present invention will become apparent to those skilled in the art upon examination of the following description with reference to the accompanying drawings.